Burn smell in car when using ac or heat, what could it be? on 1998 Toyota Avalon

I have replaced the seals in front of engine but not behind it. Don't see oil leakage, but the smell is terrible when I have heat or air on. I don't seem to get odor when heat/ac not in use.

Asked by for the 1998 Toyota Avalon
poss exh leak,have a muffler shop ck for you
See if oil is seeping from the valve cover on to the exhaust manifold.